The invention concerns an improved process for the preparation of 3-amino-1-hydroxypropane-1,1-diphosphonic acid in which the amino group may be substituted by lower alkyl radicals.
The preparation of 3-amino-1-hydroxypropane-1,1-diphosphonic acid by reaction of .beta.-alanine with a mixture of phosphorous trichloride and phosphorous acid in the presence or absence of an organic diluent is known from the German Patent Specification No. 21 30 794. However, yellow-red by-products in the form of amorphous phosphorus-oxygen compounds of unknown structure are formed in this reaction and their separation, particularly in the presence of chlorobenzene, is very costly. Another factor to be considered in the large scale preparations is the dangerously high flammability of these phosphorus compounds.
It is also known that 3-amino-1-hydroxypropane-1,1-diphosphonic acid can be prepared by reacting .beta.-alanine with a mixture of phosphorous acid and POCl.sub.3, with subsequent hydrolysis of the reaction mixture, to avoid these disadvantages. Among other drawbacks, however, this process is unsatisfactory with respect to the yields.